There exists a large number of molecules capable of attaching a zero or near-zero energy electron to form a negative ion. This ion may be the parent negative ion, or a fragment atomic or molecular ion formed via dissociative attachment. Energetically, the curve-crossing between the lowest neutral and negative ion states is made possible by the fact that the electron affinity of some atomic (e.g., F, Cl, Br, I) or molecular (e.g., CN) component is comparable to the dissociation energy of the neutral target. From symmetry considerations, the neutral-ion transition at zero electron energy often involves a transition between states of the symmetric irreducible representation, except possibly the halogen molecules. As such, the energy and symmetry requirement fulfill the s-wave threshold law in which the attachment cross section is .sigma..sub.A (E).apprxeq.E.sup.l-1/2, where l is the angular momentum component of the captured electron, and E the electron energy. For l=0(s-wave) attachment, A. Chutjian and S. H. Alajajian, Phys. Rev. A 31, 2885 (1985), were the first to demonstrate for the molecular case that the attachment process diverges as .sigma..sub.A (E).apprxeq.E.sup.-1/2. This s-wave attachment divergence had been studied earlier in nuclear physics by others in thermal neutron capture by light nuclei. See E. P. Wigner, Phys. Rev. 73, 1002 (1948); H. A. Bethe, Rev. Mod. Phys. 9 69 (1937).
In an effort to utilize the divergent, zero-energy cross sections in SF.sub.6 and the chlorohalocarbon compounds, a technique was disclosed by A. Chutjian, et al., U.S. Pat. No. 4,649,278, to focus an electron beam into an electrostatic mirror, where at the point of "reversal" the longitudinal electron energy was theoretically reduced to zero. In practice, only a fraction of electrons actually reach absolute zero energy at the point of reversal because of the lateral (transverse) velocity acquired in the electron gun. The beam of reversed electrons resembles a water fountain having a spray of finite vertical velocity and limited diameter up to the reversal region and a return spray of greater and spreading diameter. This spreading of the return flow of electrons results from a lack of shaped electrodes for electrostatic confinement in the reversal region where the electron beam achieves near-zero energies. Thus, after introduction of a beam of thermal-attaching molecules, negative ions were extracted from the collision center via the s-wave, dissociative attachment state, but no efforts were made in this earlier development to optimize electron current, reversal geometry, and extraction efficiency; nor were provisions made to minimize the transverse energy spread of the electrons at the reversal point.
In order to use this reversal electron attachment technique for ionization and ion-extraction of trace species at very low electron energies, the problem is to generate a large density of thermal electrons in the reversal region where attachment to molecules of extremely low concentration is to take place.